Which correctly summarizes the trend in electron affinity?
A. It tends to be very high for group
B. It tends to be more negative across a period.
C. It tends to remain the same across periods.
D. It tends to be more negative down a group?
The Correct Answer and Explanation is:
The correct answer is B. It tends to be more negative across a period.
Explanation:
Electron affinity refers to the energy change that occurs when an atom in the gas phase gains an electron, often expressed in terms of energy released (exothermic) or absorbed (endothermic). When an atom gains an electron, a negative ion forms. The more negative the electron affinity value, the more strongly an atom attracts additional electrons, meaning it has a greater tendency to gain an electron.
- Trend Across a Period:
As you move from left to right across a period in the periodic table (from alkali metals to noble gases), electron affinity generally becomes more negative. This increase in negativity is due to the greater nuclear charge (more protons) in the atoms of elements farther to the right in a period. A greater nuclear charge exerts a stronger pull on added electrons, increasing the tendency for the atom to accept electrons. For example, halogens in Group 17, like chlorine, have highly negative electron affinities because they need only one more electron to achieve a stable, noble-gas electron configuration, making them very receptive to gaining an electron. - Trend Down a Group:
Moving down a group, electron affinity becomes less negative. As atomic size increases (due to the addition of electron shells), the added electron is further from the nucleus and experiences less nuclear attraction, making it less energetically favorable to gain an electron. For instance, fluorine has a more negative electron affinity than iodine because, in fluorine, the added electron is closer to the nucleus and more tightly bound. - Exceptions:
Certain elements, like noble gases, have positive electron affinities because they have full valence shells, making them stable and generally uninclined to attract additional electrons.
In summary, option B is correct because electron affinity generally becomes more negative across a period due to increased nuclear charge and a stronger attraction for additional electrons, enhancing an element’s tendency to accept an electron as you move from left to right.